Measuring the Well

Techniques for measuring water levels are well established and have remained virtually unchanged for the last 40 years. Steel measuring tapes are the tool of choice; they provide uniformly accurate measurements at all depths. To prevent the tape from coiling, a lead weight is attached to the end of the tape with copper wire. The weight pulls the end of the tape downward and reduces contact with the discharge pipe, casing, and pump equipment in the well. If the tape snags on a downhole obstacle, the lead weight can break free and fall harmlessly to the bottom of the well. The 3/8-inch (1-cm)-wide steel tape can then be easily retrieved from the well. Because steel tapes do not stretch and are unlikely to become entangled with downhole equipment, they are preferred over other measuring devices.

Because most of the measurements are taken in irrigation wells, access to the water surface is commonly limited by pumping equipment (fig. 2).

Figure 2--Water well showing pumping equipment.

Most pump installations provide access to the water, either through metal tubes, threaded plug holes, or open slits in the wellhead base. These openings are generally less than 1 inch (2.5 cm) in diameter. Many times simply getting the measuring tape into the well casing through these small openings is challenging because the tape must make several sharp bends. Once inside the casing, the tape must snake alongside downhole equipment to reach the water surface (fig. 3).

Figure 3--Diagram of the downhole portion of the water well.

The depth from the surface to the top of the water is determined by subtracting the length of tape immersed in water from the total length of tape lowered into the well. This is something like checking the oil in a car. The height of the oil on the dip stick is a direct measure of the level of oil in the crankcase. However, unlike oil, water on a steel tape can be difficult to see. To improve the contrast between wet (below the water level) and dry (above the water level) tape, a thin coating of blue carpenter's chalk is applied to several feet at the end of the tape. Once chalked, the weighted tip of the tape can be coaxed through an access hole and slowly lowered between the outer casing and discharge pipe to a predetermined depth derived from the previous year's measurements (fig. 3). The light-blue chalk on the end of the tape changes color when wet, making the water line easier to see.

After reaching the measurement depth, the tape is retrieved and carefully examined for the water line. Sometimes establishing exactly where the chalk changes from light blue (dry) to dark blue (wet) is difficult; wet or grimy spots on the tape above the water line can interfere with finding the water line. These smudges or wet areas result from tape contact with the casing above the water line. In most cases, the water line is obvious, with no trace of moisture above that line. Depth to water from the ground surface is calculated by subtracting the water-line measurement from the measurement depth (amount of tape lowered into the well).

If a water line cannot be determined or the calculated change in water level is not consistent with the local trend, the well must be remeasured. The local trend is the average change in water levels from the previous year in nearby wells sampled in the same aquifer. A water-level measurement is considered good when the water line can be determined with confidence.